1. Field of the Invention
The invention relates to a steering device that is equipped with an electric motor so as to provide steering assist force for the pivoting operation of a steering handle.
2. Description of the Related Art
An electric power steering device is equipped with an electric motor so as to provide steering assist force for the pivoting operation of a steering handle, and adjusts the steering assist force by controlling the electrification of the electric motor. Such an electric power steering device is supplied with source power from a vehicle-mounted electric power source device. However, in the case where an abnormality occurs in the vehicle-mounted electric power source device, the electric power steering device cannot appropriately perform the control of the electric motor. Therefore, for example, an electric power steering device described in Japanese Patent Application Publication No. 2003-312510 (JP-A-2003-312510) is equipped with a construction that multiplies an assist command value by a gain that gradually lessens from 1 to 0 as the power source voltage of the vehicle-mounted electric power source device approaches a set value that is regarded as an abnormality. By this construction, it is intended to reduce the electric power consumption of the vehicle-mounted electric power source device and therefore maintain the steering assist function for a longer time.
Generally, in the electric power steering devices, the assist command value is computed so that the steering assist force increases as the vehicle speed becomes lower. Therefore, even with the construction that multiplies the assist command value by a gain commensurate with the power source abnormal level, a large amount of electric power is consumed from the vehicle-mounted electric power source device in the case where a static steering operation is performed. That is, the service life of the vehicle-mounted electric power source device is greatly reduced in exchange for obtaining easy or comfortable operation.